Third-generation cellular telephony uses either Code Division Multiple Access (CDMA) for the radio interface or Time Division Multiple Access (TDMA). With CDMA, a user's information bits are spread over an artificially broadened bandwidth by multiplying them with a pseudorandom bit stream running several times as fast. The pseudorandom bit stream is known as a chipping or spreading code. The users occupy the same frequency bands at the same time, but each interaction is multiplied by a different spreading code and when the signals are de-spread, the only one that comes through intelligibly is the one whose code was used by the de-spreader. The others simply add to the background noise level.
The spreading function is applied in two phases. An initial channelization code spreading, determining the occupied bandwidth of the radio signal, is followed by a scrambling code spreading. The scrambling code is used to distinguish different user equipments at the base station's receiver and to distinguish multiple cell sites in the user equipment's receiver.
Thus, the signals transmitted in an exemplary CDMA system can be formed as follows. An information data stream to be transmitted is first multiplied with a channelization code and then the result is multiplied with a scrambling code. The multiplications are usually carried out by exclusive-OR operations, and the information data stream and the scrambling code may have the same or different bit rates. Each information data stream or channel is allocated a unique channelization code, and a plurality of coded information signals simultaneously modulates a radio-frequency carrier signal.
At a user equipment, such as a mobile station or other receiver, the modulated carrier signal is processed to produce an estimate of the original information data stream intended for the receiver. This process is known as demodulation. The composite received baseband spread signal is commonly provided to a rake processor that includes a number of “fingers”, or de-spreaders, that are each assigned to respective ones of selected components, such as multipath echoes or images, in the received signal. Each finger combines a received component with the scrambling sequence and the channelization code so as to de-spread the received composite signal.
With the introduction of High Speed Downlink Packet Access (HSDPA) and increasing R99 Data Channel (DCH) traffic, the limiting factor in many cells is assumed to be the number of codes in the downlink code tree. HSDPA improves system capacity and increases user data rates in the downlink direction. Already with heavy packet traffic or in well-confined cells, it is believed that DCH traffic alone can reach code-limited scenarios and it becomes even worse when HSDPA is introduced.
To combat code limitation a number of actions may be taken. More carriers may be planned for; a higher order sectorization may also solve the problem. In addition, in the Third generation Partnership Project (3GPP) Release 6, a new channel, called Fractional Dedicated Physical Channel (F-DPCH), is introduced to share codes among HSDPA data-only users for a more efficient management of codes. However, without RBS/site impacts, the only solution is to deploy a secondary scrambling code
A secondary scrambling code (or several secondary scrambling codes) pushes the code limitation away, but there is also a negative effect: an increase of the intra-cell interference due to non-orthogonality.
With a secondary scrambling code, the downlink transmission is already from the beginning non-orthogonal, meaning that the downlink transmission will consume more power to combat the interference from other scrambling codes than the case that if they are from the same scrambling code. Consequently, a code shortage problem could potentially, as soon as the secondary scrambling code is activated, become an equally large power shortage problem. Studies show that a secondary scrambling code becomes applicable in some scenarios but degrades the performance in other scenarios.